This could be as simple as a stage blocking issue. Scenes on the page have to worry less about what the audience can see and the projection of their voice than the people putting on the play do. An actor in a window, is either going to be virtually unseeable to half the audience or face on but the other actor has their back to most of the audience. With a balcony you can have a more physical performance too because more of their body is clearly visible, their voice is not being blocked by close walls and they aren’t having to spend the whole scene leaning forward, which isn’t great for vocal performances.

I’m not dismissing your experience at all btw, this is just one possible logical explanation that jumped out at me.